The present invention was conceived in connection with monitoring the oil, water and gas production of wells involved in an oilfield thermal recovery project. While the scope of the invention is not limited to that environment, as is made clearer below, it is appropriate to begin by addressing the problems associated with such monitoring.
Thermal processes are commonly employed in recovering oil from oil field reservoirs containing heavy, viscous oil. By introducing heat into the reservoir, the viscosity of the oil is reduced, so that its mobility is greater and it can more easily be produced. In some of these processes, steam is injected into the reservoir. In others, combustion is initiated in the reservoir adjacent an injection well; air is then injected through the well to maintain the combustion and cause a fire front to slowly advance toward a production well. In both cases a pressure drive is applied to force fluids toward the production well, through which they are produced.
The production streams issuing from the wells are multi-phase in nature. They normally comprise oil, water and gases. The relative quantities of these components of the production stream vary over time, sometimes markedly and quickly.
For a variety of reasons, well operators need to be able to accurately determine, on an on-going basis, the relative proportions and the mass rates of each of the oil, water and gas. However, this is not easily accomplished.
If one retains a batch of produced fluid in a tank, free water will readily settle out and form a discrete layer. The height of the layer can be determined with a tape gauge and the volume and mass calculated. Also, most of the free gas will break out and leave the tank through an overhead line. The flow rate of this gas stream can accurately be determined using a conventional flow meter.
After the free gas separation, there remains an intermediate layer containing oil, gas and water in an emulsified form. The components of the emulsion do not readily separate to facilitate their measurement. To further complicate the matter, the relative quantities of the components vary significantly with time, making it desirable to monitor their proportions on a virtually continuous basis if any reasonable deg.ree of accuracy is required.
In actual oilfield practice, the most commonly applied technique for establishing the composition of the emulsion involves taking grab samples and centrifuging them to ascertain the "cuts" or proportions of the components.
However, there are problems associated with sampling, including:
sampling is often non-representative and therefore inaccurate; PA1 the procedure is commonly practiced manually and thus there will typically be a time lag between taking the sample and having the readings in hand. PA1 the cuts of oil, water and gas making up the emulsion; and PA1 the mass flow rates of the components. PA1 a vessel which is adapted to receive an incoming feedstock, comprising oil, gas and water in single phase and emulsion form and has outlet lines for producing an overhead gas product and an underflow liquid-containing product; PA1 means associated with the vessel for causing centrifugal and gravity separation of the feedstock components into gas, emulsion and free water layers, which are temporarily retained as a batch; PA1 means for measuring, for the batch, the mass of the column consisting of the free water and emulsion layers; PA1 means for measuring the time taken to accumulate the batches; and, PA1 means for measuring the overhead gas component production rate. PA1 a common return plate means (which can be the wall of the vessel) which is associated with a generally linear array of discrete active plates. The active plates are electrically insulated from each other and the feedstock, preferably by enclosing them in an electrically insulating closed tubular shell, and are arranged so as to be capacitively coupled with the return plate means by the fluid extending between them; PA1 the linear array of electrically insulated plates is adapted to extend vertically in the column of fluid through a vertical interval which will be intersected by each of the gas/emulsion and emulsion/free-water interfaces; PA1 the assembly further includes: (1) means for directly charging and discharging each of the active plates individually, so that the frequency of the applied potential varies with the dielectric constant of the fluid extending between the active plate involved, which is a first terminal, and the return plate means, which is the second terminal, and producing variable frequency signals indicative of said dielectric constant; and (2) means for collecting the individual signals resulting from the activation of the plates and determining for each such signal a value indicative of the dielectric constant of the fluid extending between the active plate involved and the return plate means. More specifically, the means for individually charging and discharging the plates comprises: a plurality of discrete oscillator circuits, equal in number to the Number of active plates and each positioned on or adjacent to (collectively referred to as "close to") an associated active plate, and means, such as parallel shift registers, for selectively and sequentially enabling the permanently connected oscillator circuits. The frequency signals produced by the individual oscillating circuits are measured and analyzed by a microprocessor. PA1 the capacitance probe assembly involves a multiplicity of linearly arranged active plates, each of which, when electronically activated, cooperates with the return plate means to produce frequency signals indicative of the dielectric constant of the thin slice of fluid extending between them. The microprocessor measures the so-produced individual signals and compares them, to locate the height. levels at which the values diverge from specific values derived from the known fluid components, thereby indicating an interface. Stated otherwise, the system involves using a vertical line of capacitors, operating individually, to establish a dielectric constant profile of the vessel contents to thereby identify and locate the free water/emulsion and emulsion/gas interfaces. With this information, the vertical extent of the free water and vertical extent of the emulsion layers can be determined. Since the cross-sectional area of the vessel chamber is known, the volumes of each of the two liquid-containing layers can be determined; PA1 the system further includes means for measuring the total mass of the liquid-containing column formed by the batch. Since the specific gravity of the free water is known and the height and cross-sectional area of the free water layer are now known, the mass of the free water layer can be determined, thereby yielding the mass of the emulsion layer by subtraction. As the mass and volume of the emulsion layer are now known, a value indicative of the specific gravity of the emulsion can be determined; PA1 the capacitance values derived from the emulsion can now be compared against previously assembled capacitance data for known ratios of the water in oil emulsion. Comparison of the emulsion values against the reference data will yield the approximate oil/water ratio of the emulsion and permit of calculation of the approximate water volume ratio. PA1 using the approximate volume ratio of water, the volume of gas in the emulsion can be computed. With the recomputed values, accurate values of the mass of water and mass of oil in the emulsion can be determined.
There has therefore long existed a need for an in-line assembly that could automatically, virtually continuously and accurately establish:
As a first step in this direction, the present assignee has developed a metering separator capable of monitoring the mass flow rate of the liquid contained in an oilfield production stream. This separator is described in U.S. Pat. No. 4,549,432.
In connection with this separator, the in-coming feed stream is delivered tangentially into the vessel, so that the fluid is caused to swirl. Most of the gas breaks out, forms a central vortex, and leaves the vessel chamber through an overhead outlet line. The flow rate through this line is measured with a meter. The remaining fluid is temporarily retained as a batch in the vessel chamber. Free water quickly settles and forms a bottom layer. A layer of gassy water-in-oil emulsion accumulates above the free water. Together these bottom two layers form a liquid-containing column. The free gas, of course, forms a third layer above the column. A differential pressure transducer is used to monitor the accumulating head of the column. The time taken to accumulate the batch is also measured. When the head reaches a pre-determined value, control means close a fill valve, open a dump valve and the batch is quickly discharged from the vessel. Back-pressure is maintained on the gas outlet line to push the fluid out during dumping. The signal from the differential pressure transducer initiates closure of the dump valve when the head reaches a pre-determined low. A microprocessor keeps track of the number of dumps and calculates the mass flow rate of the liquid passing through the unit, by using the head measurement, the time measurements, and the known internal cross-section area of the separator.
However, up until the development of the present invention, the relative cuts of the components in the dumped column were determined by sampling, accompanied with the shortcomings previously referred to.
Thus the development of the present invention was initiated to attain the end of being able to automatically establish, for a batch, the heights of each of the free water and emulsion layers and the cuts of water, oil and gas forming the emulsion. With this information, taken in conjunction with other known information, specifically the cross-sectional area of the vessel chamber and the known specific gravity of the components, it would be possible to compute the mass flow rates of each of the oil, water and gas.
The present system incorporates the use of capacitance to establish a measure of the dielectric constant of the fluid extending between plates. When the fluid between the plates is an emulsion, the measured capacitance will vary depending upon the relative proportions of the emulsion components. Generally, capacitive devices have heretofore been used in oilfield applications for establishing oil/water ratios in conjunction with a two component fluid. To our knowledge, few systems cope with the presence of a third component, gas, in the emulsion. One such system, described in U.S. Pat. No. 4,289,020 issued to PAAT, used a nuclear device to determine the density of an emulsion. This measurement enabled the associated apparatus to calculate the oil/water ratio of the emulsion. But it is expected that this device cannot cope with free water. In addition, capacitive devices have been disclosed in the prior art for the purpose of locating interfaces in a two phase column of fluid (e.g. see U.S. Pat. No. 4,503,383, issued to Agar).